Understanding the magnitude and temporal structure of variations in solar
ultraviolet irradiance is a key component of understanding the Sun as a
variable star and is critical to understanding solar forcing of the Earth's
upper and middle atmosphere. We have used indices derived from the daily,
spatially resolved, magnetograms taken at the Mount Wilson Observatory to
develop models of UV observations. These magnetograms, which have been
taken in the magnetically sensitive FeI 525.0 nm line on a daily basis since
1968, offer the possibility of long term modeling of solar UV variations,
including periods for which satellite observations are unavailable or
unreliable. One index used in this modeling is the "Magnetic Plage Strength
Index" (MPSI), which is defined as the sum of the absolute magnetic fields
of all pixels with magnetic strength between 10 and 100 gauss and is
associated with the strong fields of plage/facular regions. We have found
the MPSI to describe reasonably well both short and long term variations in
the UV irradiance observations from the Solar Mesosphere Explorer, Nimbus-7,
NOAA9, and Upper Atmosphere Research Satellites with correlation coefficients
of from 0.93 to over 0.98. However, there remains a substantial fraction of
solar UV variations which appears not to be associated with these strong
fields.
In this paper we describe our efforts to improve on the MPSI
model of these satellite observations by using different field
strength limits for a plage index, (2) creating new and improving on old
magnetic indices by including in the model surface magnetic features with field
strengths lower and higher than associated with the MPSI, and (3) correcting
the Mount Wilson indices for certain changes in the observation protocol.
Since the bulk of the magnetogram pixels have field strengths between 0.5 and
2.0 gauss as measured at 525.0 nm, and since pixels with field strengths
between 2.0 and 10 gauss may be associated with active network, we think,
and our results so far confirm, that indices based on fields
lower than 10.0 gauss will significantly improve modeling of UV variations.

The author(s) of this abstract have provided an email address for comments about the abstract: parker@astro.ucla.edu